From the UDN licensee:
I put together a draft PR of something that should work. As I mentioned in the PR, this works as expected with one caveat; in SPinComboBox::Construct we are hardcoding the tooltip to be "Select enum values from the list." This would either need to be changed to a different, more generic hardcoded value or it would need to be reworked into something that can be driven by the owner of the combo box.
I doubt that you'll actually take the PR since there likely would need to be some other cleanup (e.g. deleting SNameComboBox.h/cpp) and you might want to restructure it to make more sense, but this should be most of the work of actually getting the display to work.
At this point, the value will have been reset but it will not look any different to the user. If they compile the BP, the node will refresh back to the broken-looking state from step 3. Until this happens, the node is basically unusable since part of the editor sees Curve as the selected value and therefore will not send the ComboBoxSelectionChanged when selecting Curve again but the underlying system has reset back to nothing. This is only an issue because there is only one value in the list, but the same thing would happen if the user attempted to select the same value in a list with multiple entries.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-195555 in the post.